Magnetic resonance imaging (MRI) has become increasingly important in the detection, diagnosis and monitoring of diseases due, for example to the flexibility of the method, and the detail of the images produced. For example, this non-invasive technique produces 2- and 3-D images with sub-mm spatial resolution, without the use of ionizing radiation.
The majority of the 1.5 million MRI scans presently performed in Canada each year involve the use of contrast agents; compounds containing paramagnetic metal ions which enhance the contrast, for example, between healthy and diseased tissue. Contrast agents operate by altering the local magnetic field strength of a tissue and changing the relaxation times (T1 and T2, in s) of the surrounding water protons. The effectiveness of a contrast agent is described by its relaxivities, r1 and r2 (s−1·mM−1), where r1=(1/T1)/c and r2=(1/T2)/c, and c is the concentration of the contrast agent in a given media.
High values of r1 and r2 are useful. Optimizing relaxivity involves maximizing several parameters. For example, relaxivity increases as the rate of molecular tumbling (τR) decreases, and as the rate of water exchange increases. Relaxivity also increases with the number of coordinated water molecules, q. These parameters are depicted in FIG. 1.
There are eight small-molecule contrast agents presently authorized for use in Canada. All are made up of a single GdIII ion and either a chelating diethylenetriaminepentaacetate (DTPA5−)-based ligand, or a macrocyclic 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA4−)-based ligand (Scheme 1). The MnII-based agent Teslascan™ based on the ligand dipyridoxal diphosphate (DPDP2−; Scheme 1) has also been utilized, but was recently removed from the market.

The major drawback to the currently approved MnII- and GdIII-based agents is the biotoxicity of the free metal ions. MnII dissociates quickly from the DPDP2− ligand, and has neurotoxic effects. The ion is readily transported across the blood-brain barrier, where it accumulates and causes Parkinson-like symptoms, including tremors and muscle stiffness.1 GdIII is strongly associated with nephrogenic systemic fibrosis (NSF), an acquired disorder in patients with suppressed renal function involving a hardening of the skin, the muscles of the heart, and the walls of organs such as the liver.2 There is thus ongoing research into safer, more effective alternatives to the approved contrast agents.
Dual-property imaging agents; for instance, bi-modality imaging species combining MRI properties with fluorescence3, or ‘theranostic’ agents which both image and deliver medicinal benefits4 have also been disclosed.
The most common techniques used to study the dyes comprising a fluorescent group are co-facial microscopy, optical imaging and fluorescence microscopy. Prior to being used in such techniques, the fluorescence properties of the individual compounds can be first studied with a spectrofluorometer.
In vitro cellular fluorescence imaging is typically carried out using a confocal microscope. In this respect, a confocal microscope is usually used to measure the intensity of the emitted fluorescence signal and create a digital image. In contrast to widefield microscopy, confocal microscopy has an increased optical resolution and contrast. It uses point illumination and a spatial pinhole in front of the detector to restrict passage of light that comes from the plane of focus. Out-of-focus light from specimens that are thicker than the focal plane is thereby eliminated. The thickness of the focal plane is largely determined by the emission wavelength and the numerical aperture of the object lens. In confocal microscopy, only one point in the sample is illuminated at a time. In order to take 2D or even 3D images one must therefore scan over the specimen. Post-processing of images taken by confocal microscopy makes it possible to depict and quantitate the obtained signals.
In MRI-guided fluorescence tomography (in vivo dual-imaging), magnetic resonance (MR) image sequences are collected simultaneously with fluorescence signals using a MR-coupled diffuse optical tomography system. Image reconstruction is generally performed multiple times with varying abdominal organ segmentation in order to obtain an optimal tomographic image. This has been used to follow the treatment/progress of cancer tumours since the fluorescence is greater in the diseased tissue. For example, Samkoe et al. disclose a MR-guided diffuse optical tomography system wherein the fluorescence system is made up of a CW laser (690 nm), rotating source coupling stage, 16 spectrometers and 16 long, bifurcated source-detector optical fibers which are channeled through a conduit in the wall and couple directly into the bore of the MRI5.
Complexes of the bis-amine macrocycles depicted in Scheme 2 have been investigated as imaging agents. However, the biological activity of these compounds is complicated by their ability to act as superoxide dismutase mimics.

[1+1] Schiff-base macrocycles can be formed by the metal-templated condensation of a diketone and a diamine. The MnCl2-templated formation of macrocycles L1 and L2 is shown in Scheme 3, resulting in complexes 1 and 2 respectively. These two complexes have been previously reported, and studied as building blocks for the synthesis of magnetically interesting chains and clusters.6

The biodistribution of a small-molecule, non-targeted contrast agent typically follows the sequence: (1) intravenous injection; (2) distribution in the blood; (3) distribution in the extracellular space; and (4) pathway through the excretory organs. Such agents are known as extracellular fluid (ECF) agents.
From the extracellular space, an agent may or may not be taken up into the intracellular environment of an organ or organs, depending on its structure. Uptake may be active or passive.
There are nine small-molecule contrast agents presently approved for use in North America (Table 1)7. Magnevist™, Omniscan™, OptiMARK™ Dotarem™, Prohance™, and Gadavist™ are ECF agents, and are used for whole-body and CNS imaging. They are excreted primarily through the kidneys with an average half-life of 1.5 hours.
The agents Primovist™ and Multihance™ are also ECF agents, but their structures contain benzyl groups, so they are taken up by hepatocytes during excretion: thus are useful as liver-specific imaging agents.
In contrast, the approved agent Ablavar™ selectively binds to the blood protein serum albumin, and thus remains in the vascular system as a blood pool agent, suitable for imaging vasculature.
There are a number of parameters which are selected for a given MRI scan, including8: magnetic field strength; RF (radiofrequency) pulse timing; TE: echo time (ms); TR: repetition time (ms); RF pulse amplitude; and gradient timing and amplitude. MRI sequences can be classified by the type of sequence (such as spin-echo, gradient-echo, or inversion recovery) but are more commonly described by the image weighting:
T1 weighted: short TE and short TR
T2 weighted: long TE and long TR
Proton density (PD) weighted: short TE, long TR.
A given tissue is usually evaluated by multiple sequences, collectively known as the MRI protocol. The same contrast agent may, for example give rise to different tissue enhancement under a different sequence.